LiNiO2-based cathode active materials are promising candidates to replace LiCoO2 in commercial rechargeable batteries. The advantages of such an active cathode are summarized in the below.
(1) Price and Availability of Raw Materials:
Increasing quantities of the world production of Co are used for the production of LiCoO2. This share will further increase as the actual growth of the Li-battery market and particularly the trend of implementing larger Li-batteries continues. Since Co resources are limited, its price is expected to rise. On the other hand, the price of Ni is low, and its much larger market is expected to be able to easily adsorb demand from a growing battery industry.
(2) Capacity:
The reversible capacity of doped LiNiO2 is approx. 200 mAh/g when charged to 4.3V, exceeding the capacity of LiCoO2 (approx. 165 mAh/g). Therefore, despite a slightly lower average discharge voltage and slightly lower volumetric density, commercial cells with LiNiO2 cathode have an improved energy density.
However, there are severe problems that hinder the wide and successful implementation of LiNiO2-based cathode active materials as described in below.
(A) Price:
It is generally accepted that LiNiO2 of high quality cannot be prepared by such simple methods as are used for LiCoO2 production, i.e., simple solid state reaction of a Co precursor with LiCoO2. Actually, doped LiNiO2 cathode materials in which an essential dopant is cobalt and further dopants are Mn, Al, etc. are produced on a large scale by reacting lithium precursors such as LiOH*H2O with mixed transition metal hydroxides in a flow of oxygen or synthetic air (i.e., CO2 free). Also, additional steps such as an intermediary washing or coating further increase the cost of such processes.
(B) Safety, Gassing, Gelation and Aging:                Safety: the implementation of LiNiO2 has been delayed by concerns about the safety of LiNiO2 batteries. The safety of the cathode powder can be increased to some extent, for example, by modifying the composition of the cathode powder or optimizing the morphology. Furthermore, the safety of batteries can be improved by battery design, electrolyte modifications, etc.        Storage properties: the commercial implementation of LiNiO2 has particularly been delayed due to poor storage and abuse properties. A severe problem, which has not been solved yet, is the evolution of an excessive amount of gas during storage or cycling. Excessive gas activates the safety switch to shut down a cylindrical cell and also causes a polymer battery to swell. The inventors of the present invention found that there is a correlation between the content of soluble base and the excessive gas evolution, and particularly that the amount of Li2CO3 (as determined by pH titration) has a close relation to the amount of gas evolved during storage.        Processing: another problem of LiNiO2 involves the stability of the cathode material (when exposed to air and humidity, LiNiO2 deteriorates rapidly) and the gelation of slurries (due to a high pH, the NMP-PVDF slurry starts to polymerize). These properties cause severe processing problems during battery production.        
Many prior arts focus on improving properties of LiNiO2-based cathode materials and processes to prepare LiNiO2. However, the problems of high production cost, swelling, poor safety, high pH and the like have not been sufficiently solved. A few examples will be illustrated in below.
U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo) discloses a wide range of compositions including nickel-based and high-Ni LiMO2, the materials having high crystallinity and to be used in Li-ion batteries in EC containing electrolyte. Samples were prepared on small scale, using LiOH*H2O as a lithium source. The samples are prepared in a flow of synthetic air being a mixture of oxygen and nitrogen, free of CO2.
U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a doped LiNiO2 substantially free of lithium hydroxide and lithium carbonate. For this purpose, transition metal hydroxide and LiOH*H2O as a lithium source are employed and heat treatment is performed under an oxygen atmosphere free of CO2, additionally with a low content of H2O. An excess of lithium “evaporates”; however, “evaporation” is a lab-scale effect and not an option for large-scale preparation.
U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita) discloses a process to prepare LiNi1−xMnxO2 doped by Mn, x<0.45, in which the manganese source is Mn-nitrate, and the lithium source is either lithium hydroxide or lithium nitrate.
U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita) discloses a process to prepare LiNi1−xMnxO2 by a two-step heating, involving pre-drying, cooking and the final heating. The final heating is done in an oxidizing gas such as air or oxygen. This patent focuses on oxygen. The disclosed method uses a very low temperature of 550˜650° C. for cooking, and less than 800° C. for sintering. At higher temperatures, samples are dramatically deteriorated. Excess lithium is used such that the final samples contain a large amount of soluble bases (i.e., lithium compounds). According to research performed by the inventors of the present invention, the observed deterioration is attributable to the presence of lithium salts and melting at about 700˜800° C., thereby detaching the crystallites.
WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a complicated preparation method very different from that disclosed in the present invention. This preparation method involves the use of lithium-nitrates and lithium hydroxides and recovering the evolved noxious gasses. Sintering temperature never exceeds 800° C. and typically is far lower.
U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process to prepare LiNiO2-based cathodes from lithium hydroxide and metal oxides at temperatures below 800° C.
In prior arts including the above, LiNiO2-based cathode active materials are generally prepared by high cost processes, especially in a flow of synthetic gas such as oxygen or synthetic air, free of CO2, and using LiOH*H2O, Li-nitrate, Li acetate, etc. but not the inexpensive, easily manageable Li2CO3. Furthermore, the final cathode materials have a high content of soluble bases, originating from carbonate impurities present in the precursors, which remain in the final cathode because of the thermodynamic limitation. To remove the soluble bases, additional steps such as washing, coating etc. are required, thereby increasing the cost.
Therefore, there is a strong need for LiNiO2-based cathode active materials able to be prepared at low cost from inexpensive precursors such as Li2CO3, having a low content of soluble base, showing improved properties such as low swelling when applied to commercial rechargeable lithium batteries, improved safety and high capacity.